There has been proposed a LED (light-emitting diode) projector that uses a LED as a light emitting element of a light source. The LED projector of this type includes an illumination optical system into which light from the LED enters, a light bulb having a liquid crystal display plate or a DMD (Digital Micromirror Device) into which light from the illumination optical system enters, and a projection optical system for projecting light from the light bulb to a projection surface.
In the LED projector, to increase the luminance of a projected image, light loss must be prevented as much as possible on an optical path from the LED to the light bulb.
As described in Nonpatent Literature 1, there are restrictions based on etendue that are determined by the product of an area of the light source and the radiation angle. In other words, the light from the light source is not used as projection light unless a value of the product of the light-emitting area of the light source and the radiation angle is set equal to or less than a value of the product of an incident surface area of the light bulb and a capture angle (solid angle) determined by the F number of the illumination optical system.
Thus, reducing light loss by lowering the etendue of light output from the LED is an issue that requires attention.
In the light source of the LED projector, a light source to emit a luminous flux of about several thousand lumina is required. To achieve this, a LED having high luminance and high directionality is essential.
As an example of such a light emitting element having high luminance and high directionality, as shown in FIG. 1, Patent Literature 1 discloses a semiconductor light emitting element configured by sequentially stacking n-type GaN layer 102, InGaN active layer 103, p-type GaN layer 104, ITO transparent electrode layer 105, and two-dimensional periodic structure layer 109 on sapphire substrate 101. This light emitting element, a part of which is cut out to form groove 108, includes n-side bonding electrode 106 disposed in a part of n-type GaN layer 102 in groove 108, and p-side bonding electrode 107 disposed in ITO transparent electrode layer 105. In this light emitting element, directionality of light from InGaN active layer 103 is increased by two-dimensional periodic structure layer 109. Then, the light is output from the light emitting element.
As another example of the light emitting element having high luminance and high directionality, as shown in FIG. 2, Patent Literature 2 discloses organic EL element 110 configured by stacking anode layer 112, hole-transport layer 113, light emitting layer 114, electron-transport layer 115, and cathode layer 116 having micro periodic relief structure grating 116a on substrate 111. This light emitting element has high directionality that enables setting of a radiation angle of light output from the light emitting element to be less than ±15° due to the effect of surface plasmons propagated through an interface between micro periodic relief structure grating 116a of cathode layer 116 and the outside.